legal guardianship over two nonerelated kids

No unless they are related by blood.

You could call them/raise them as siblings but time will pass and learning the truth or not reinforcing the idea they are siblings might lead to problems down the road.

A family is a family.

I remember this guy raising the daughter of a woman that he had nothing to do with while the womans-mother ( Grand-ma ) is living with him. The mother is associating with "rich guys". Clearly I could see what is going on but the mother is like some kind of insane party-girl who does not want the party to stop. It is silly. Maybe it is the fear of age and getting older.

I feel bad for the little girl.